This application is a U.S. national phase application of PCT application No. PCT/FI00/00685 filed on Aug. 11, 2000 and published under PCT Article 21(2) in English, which claims priority of application Serial No. 19991703 filed in Finland on Aug. 11, 1999.
This invention relates to a method for evaporating aqueous solutions containing ammonia, the method comprising condensation of the evaporated vapor into a liquid.
In the evaporating treatment of effluents or similar aqueous solutions containing ammonia (NH3), ammonia is evaporated along with the aqueous vapor if the pH is about 6 or more. Condensation of the formed evaporated vapor containing ammonia is troublesome, because the poorly water-soluble ammonia forms an inert gas layer on the condenser surfaces, where it hampers heat transfer. The negative impact of ammonia on the condenser operation starts at an ammonia concentration as low as approx. 300-500 ppm. For this reason, complete condensation of vapor containing ammonia requires a markedly greater temperature difference in the condenser than does condensation of pure aqueous vapor.
The poor condensability of ammonia involves a problem in evaporators whose operation is based essentially on the utilization of heat released from condensed vapor in the process. Especially in Mechanical Vapor Recompression (MVR) evaporators, in which the available temperature difference is low, or in multistage evaporating plants, which use low value waste heat, the high ammonia concentration of the vapor may stop the evaporator operation entirely.
The aqueous solutions rich in ammonia and treated by evaporation that cause problems comprise i.a. effluents from the chemical industry, the slaughtering industry and agriculture, leachate water from landfills and sludge digestion press waters from urban waste water purification plants.
Ammonia can be removed from the aqueous solution before evaporation by means of a combined stripper-scrubber, where ammonia stripping may be performed with the use of inert gas. During the stripping, the solution should have a minimum pH of 9-10, and to achieve this, lye addition to the solution is required in most cases. The treatment of contaminated effluents involves the additional problem of the stripper being clogged by solid matters precipitated onto the packing elements, and used with hot water, the stripper causes heat losses. In all these cases, the stripper and the scrubber entail extra device and operational costs and result in a complex process.
Another known method for eliminating the ammonia problem in evaporation is to drop the pH of the solution by means of acid to a value below 5, at which ammonia is neutralized in salt form. In this form, ammonia does not evaporate, but remains in the concentrate during evaporation. However, this process has the drawbacks of high acid consumption, especially when the solution has a high buffer capacity, and also of carbon dioxide being possibly released from the carbonates, requiring a gas stripper before evaporation in order to be removed. In addition, the use of hydrochloric acid in particular increases the corrosivity of the solution, whereas the use of sulfuric acid may result in detrimental gypsum deposits if the solution contains calcium.
Ammonia can also be removed from the solution by means of a biological process, in which the nitrogen of the ammonia is first oxidized into nitrate and then reduced into nitrogen gas. Nevertheless, such a process is typically susceptible to circumstances, such as water quality and any nutrients contained in the water, and thus it has poor operational reliability.
The object of the invention is to provide a new solution to the problem of evaporating aqueous solutions containing ammonia, in which the prior art in conveniences described above are avoided. The method of the invention is characterized by adding acid to the evaporated vapor emitted from the solution in order to bind the ammonia contained in the vapor into ammonium salt.
With the acid addition of the invention, ammonia neutralizes into a water-soluble salt form, which will no longer form an inert gas layer on the condensation surfaces, which would prevent heat transfer at the condensation stage. By adding acid to the evaporated vapor in an amount stoichiometrically equivalent to the ammonia neutralization reaction, the problem caused by inert gas can be totally overcome.
Hydrochloric acid, sulfuric acid and phosphoric acid, for instance, are suitable acids for ammonia neutralization. Nitric acid and some organic acids are also potential acids in this conjunction. Acid is preferably added to the evaporated vapor by spraying or atomizing.
The method of the invention is applicable to many types of evaporating plants, which may comprise one single step or many steps. The invention is especially advantageous in multi-stage evaporators operating at gradually decreasing temperatures and in MVR evaporators equipped with a compressor, such as film distillers, in which evaporated vapor compressed to higher pressure is recycled to the evaporator as heating vapor. In these plants, acid addition preferably takes place by atomizing at a suitable location of the vapor duct before the condenser, either before or after the compressor in MVR evaporators. The large heat transfer surfaces of the condenser provide a contact surface that ensures complete reaction between acid and ammonia.
The ammonium salt produced in the types of evaporators mentioned above ends in the condensate, from where it can be removed if necessary for instance by reverse osmosis, ion exchange, re-evaporation or oxidation either with break-point chlorination or ozone. These processes have reliable operation, because the condensate contains no other impurities besides ammonium salt. A condensate containing ammonium salt can also be utilized as such as a fertilizer or an industrial raw material for instance.
The invention can also be implemented with the ammonium salt produced in ammonia neutralization being recovered without reaching the condensate formed. from vapor. This is performed by conducting the evaporated vapor to a scrubber, which is supplied with acid and where the formed ammonium salt is separated e.g. with a droplet separator from the vapor flow before the vapor is condensed. If the scrubber is connected to an MVR evaporator, it is preferably placed apart from the evaporator, either on the suction side or the pressure side of the compressor. Most preferred is the use of a Venturi scrubber, which causes negligible pressure loss in the vapor flow. The ammonium salt solution separated by the scrubber may have a concentration of about 10-30%, at which it is usable as a fertilizer or an industrial raw material.
In a further embodiment of the invention, the pH of the aqueous solution to be evaporated is raised with an alkaline addition so that the ammonia is released and passes to the evaporated vapor during evaporation. In other words, the invention allows also such aqueous solutions with a low pH to be treated whose evaporation would not release ammonium normally, i.e. without this alkaline addition. Since, in accordance with the invention, the ammonia release is under control, the process provides an advantageous means for recovering ammonium from the solution while preventing the ammonium from remaining in the concentrate, where it may be detrimental. If, for instance, such a concentrate were led to the waste bed of a landfill, the ammonia would affect the internal bioactivity of the landfill.